import pandas as pd
df =pd.DataFrame({'商品': ['苹果', '香蕉', '梨'],
        '单价': [5, 5, 4],
        '销量': [100, None, None ],
        '库存': [50, None,70]})
print(df)
print(df.dropna())
print(df.dropna(axis=1))
print(df.dropna(thresh=3))
print(df.dropna(subset=['库存']))